Articles molded from polypropylene are excellent in rigidity, heat resistance, surface glossiness, etc. and hence they have been widely applied to various uses.
However, polypropylene is generally crystalline and is deteriorated in impact resistance, particularly in low-temperature impact resistance, and therefore application thereof is limited to certain uses.
For increasing the impact resistance of polypropylene, conventionally adopted are a method of adding polyethylene to polypropylene and a method of adding thereto rubber-like materials such as polyisobutylene, polybutadiene and a non-crystalline ethylene/propylene copolymer. In particular, a method of adding a non-crystalline or low-crystalline ethylene/propylene random copolymer has been used in many cases.
However, the present inventors have studied on polypropylene compositions comprising the non-crystalline or low-crystalline ethylene/propylene random copolymer and polypropylene, and found that the non-crystalline or low-crystalline ethylene/propylene random copolymer cannot improve the impact resistance so much and hence the ethylene/propylene random copolymer must be contained in the polypropylene composition in a large amount to obtain satisfactory impact resistance. When a large amount of the ethylene/propylene random copolymer is contained in the polypropylene composition, the composition can be improved in the impact resistance but seriously lowered in rigidity, heat resistance and surface hardness. On the other hand, if the ethylene/propylene random copolymer is contained in the polypropylene composition in a small amount to retain rigidity, heat resistance and surface hardness, the low-temperature impact resistance of the polypropylene composition cannot be sufficiently improved.
In place of using such non-crystalline or low-crystalline ethylene/propylene random copolymer, a trial of adding other ethylene/.alpha.-olefin copolymer to polypropylene was made to obtain a polypropylene composition having high impact resistance. For example, Japanese Patent Publications No. 25693/1983 and No. 38459/1983 disclose a composition comprising crystalline polypropylene and an ethylene/1-butene copolymer which contains constituent units derived from 1-butene in an amount of not more than 15% by mol. In addition, Japanese Patent Laid Open No. 243842/1986 discloses a polypropylene composition comprising crystalline polypropylene and an ethylene/1-butene copolymer which is obtained by using a titanium heterogeneous type catalyst. The polypropylene compositions disclosed in these publications are improved in the impact resistance and the rigidity, but they are desired to be much more improved in the low-temperature impact resistance.
Further, Japanese Patent Publication No. 42929/1988 discloses a polypropylene composition comprising crystalline polypropylene and an ethylene/1-butene copolymer which contains constituent units derived from 1-butene in an amount of 25 to 10% by weight and has an intrinsic viscosity [.eta.] of not more than 1.5 dl/g. This polypropylene composition is insufficient in the impact resistance.
Furthermore, Japanese Patent Laid-Open No. 250040/1991 describes that an ethylene/1-butene block copolymer containing constituent units derived from 1-butene in an amount of 10 to 90% by weight is used to increase impact resistance of polypropylene. However, this ethylene/1-butene block copolymer is not good in compatibility with polypropylene and insufficient in improvement of the impact resistance.
On that account, eagerly desired now is the advent of a polypropylene composition excellent in impact resistance, particularly in low-temperature impact resistance, as well as in rigidity and heat resistance.
As a result of earnest studies by the present inventors to solve the problems associated with the prior arts, they have found a polypropylene composition comprising polypropylene and a specific ethylene/.alpha.-olefin copolymer is excellent in rigidity, heat resistance and impact resistance, particularly in low-temperature heat resistance, and accomplished the present invention.